This invention is concerned with a ballast for igniting and operating two rapid start fluorescent lamps from a relatively low voltage source.
Rapid start lamps are in wide spread use at the present time because they are reliable, efficient and furnish good illumination for the energy they consume. They are oftentimes preferred to the so-called instant start types of lamps because they start and operate at lower voltages. This is accomplished by the use of rugged filaments in the lamps which are energized at all times that the lamps are operating.
The typical two-lamp rapid start circuit includes a ballast which furnishes the igniting and operating voltage in a manner known as series-sequence. The circuit provides for the starting of the lamps in sequence so that they do not require the sum of the starting voltages to start them. Once the lamps have been ignited they operate in series and a condenser is provided for achieving leading current in the circuit for preferred power factor effects.
A typical rapid start arrangement has a pair of 48 inch T-12 rapid start lamps of 40 watt rating mounted in a single metal fixture connected into a 120 volt 60 hertz source of power. The ballast has a primary winding and a secondary winding which may be connected in autotransformer or straight transformer relationship so that all or a part or none of the primary winding is used in the operation of the lamps and their starting. The second-to-ignite lamp is shunted by a capacitor so that on open circuit all of available voltage is across the first-to-ignite lamp, by-passing the second-to-ignite lamp. As soon as the first-to-ignite lamp is ignited and current flows in the circuit, the shunting capacitor acquires a voltage which is high enough to ignite the second lamp. The impedance of the second-to-ignite lamp being much lower than the reactance of the shunting capacitor, current prefers to pass through the lamp than the shunting capacitor. As a result, the effect is to exclude the shunting capacitor from the operating circuit. As will be seen from the discussion which follows, the shunting capacitor will produce undesirable effects under certain circumstances, which must be alleviated.
When both lamps are ignited the circuit includes the connected windings of the transformer and the series capacitor which played no role in the starting of the lamps other than to enable the voltage effect of the windings on open circuit to be applied across the first-to-ignite lamp. The connected windings may include the secondary winding only, the secondary and primary windings if they are connected in autotransformer relationship or the secondary winding with a part of the primary winding which is connected thereto in autotransformer relationship.
The ballasting effect, that is, the development of sufficient inductance in the circuit to prevent the lamps from passing large currents on igniting (since the lamps are negative resistance characteristic gaseous discharge devices whose impedances drop radically when they ignite) is achieved normally by loosely coupling the secondary winding relative to the primary winding. This is achieved by means of shunts and/or physical separation of the windings. The core of the transformer will usually have one or more bridged air gaps to improve wave shape and prevent saturation in the winding leg under the secondary winding where the slots are normally located.
Typical prior art patents which are directed to the field of the invention and disclose rapid start ballasts including their circuitry are:
Feinberg--Pat. No. 2,820,180 PA0 Feinberg--Pat. No. 2,900,577 PA0 Feinberg et al--Pat. No. 3,195,012 PA0 Hume--Pat. No. 3,225,255 PA0 Colliton--Pat. No. 4,185,231
Some of the techniques of these prior art patents are used in the circuitry of the invention including isolating the primary and secondary windings for safety and providing means for aiding in the starting of the lamps through the use of the grounded metal fixtures providing an additional voltage through the use of a resistor to ground in circuit with the primary winding.
Recent practice in the lighting industry has tended toward the use of lamps which are intended to save energy by being operated under conditions somewhat different than heretofore. Such conditions include lower currents which reduces the power consumption. Lamps which are built for nominal ratings of a given value are operated at lower ratings in the effort to save power.
The most popular type of rapid start lamp in use today is the 40 watt T-12 48 inch lamp and the most frequently used ballast circuit is one in which the primary winding and secondary winding are connected in autotransformer relationship so that the primary voltage is available, not only to assist in the starting of the first-to-ignite lamp but also to provide voltage for maintaining operation of both lamps in series. This type of ballast is very economical and considered relatively safe; hence when utilized with a line voltage of 120 volts it is not considered necessary to provide isolation between the primary and secondary windings. The minimum peak voltage to ground at the lamp sockets is 180 volts RMS. The capacitor which is used for the shunting circuit around the second-to-ignite lamp is almost invariably a 0.05 microfarad capacitor because the capacitance of 0.06 microfarad has been established by insurance testing laboratories as the maximum safe value. Accordingly there need not be a bleed resistor connected across a capacitor of this capacitance or less because the maximum discharge from such a capacitor is not considered to be harmful.
The energy saving lamps of recent times operate at lower wattages of 34 and 35 and are adapted for use with the same ballasts that have been designed for 40 watt lamps. Their arc voltages are less than those of the conventional 40 watt rapid start lamps so they consume less power. Some manufacturers of ballasts have constructed ballasts specifically for the lower wattage lamps but the construction differs very little from that of the ordinary two lamp series-sequence rapid start ballasts for 40 watt lamps.
The lamps with which the invention is concerned are intended for use under circumstances which normally would call for 40 watt lamps and in fact prior to the present period of time 40 watt lamps would have been used. The lamps involved are of a new type that have a power rating of 32 watts. These lamps are of smaller diameter than the normal T12 40 watt lamp, being known as T-8 lamps which are 1 inch in diameter. They are of the rapid start type and are at the present time being manufactured by GTE Products Corporation, Sylvania Lighting Center, Danvers, Mass. and are offered under the trademark "Octron". The manufacturer states that these lamps have an efficiency of 90 lumens per watt compared with 78 lumens per watt for most ordinary fluorescent lamps. The color is considered desirable in that the lamps have a Color Rendering Index (CRI) of 75 compared to CRI's of 62 and less for other fluorescent lamps.
Certain problems arise in connection with igniting and operating these lamps which will be described presently and which are solved by the invention.
The 32 watt rapid start lamps require higher igniting and operating voltages than the ordinary 40 watt lamps; hence the usual 40 watt rapid start ballast for two sequence-start lamps will not be suitable for igniting the 32 watt lamps or maintaining them in operation. The igniting voltage for the 32 watt rapid start lamp is 300 volts while the igniting voltage for the normal 40 watt rapid start lamp is substantially less. A conventional two lamp 40 watt rapid start ballast will not ignite the 32 watt "Octron" lamps. The resulting requirements for voltage and capacitance of the shunting capacitor produce conditions which result in a transformer that must generate higher voltages and a circuit which will consequently provide higher igniting voltages.
The 40 watt rapid start ballast of conventional construction utilizes a 0.05 microfarad shunting capacitor to aid in starting the second-to-ignite lamp. The economy of not being required to connect a bleed resistor across a capacitor of this capacitance together with the fact that adequate starting voltage for the second-to-ignite lamp of about 250 volts RMS is developed in the capacitor has made the 40 watt ballast very popular. In addition, the currents in the series operated lamps are well-balanced and there are no high current pulses or spikes in the second-to-ignite lamp during operation which could damage that lamp.
Aside from the fact that higher voltages are needed than generated in the normal 40 watt rapid start ballast for two lamps, the conventional circuitry is not suitable for "Octron" lamps because of an important reason. The manufacturer of the lamps specifies the need for a 0.1 microfarad capacitor shunting the second-to-ignite lamp in order to achieve a sufficient voltage across the capacitor to ensure ignition of the second-to-ignite lamp. At the same time the manufacturer specifies the maximum peak current that the 32 watt rapid start lamp will pass without damaging the cathodes.
In the case of the 32 watt T-8 fluorescent lamp mentioned above a problem has arisen in the use of the 0.1 microfarad capacitor in that a very high current spike accompanies the ignition of the second-to-ignite lamp on each half cycle of the alternating current. This spike is apparently caused by the fact that the T-8 lamp ignites so rapidly each half cycle that whatever charge has been accumulated in the shunting capacitor during the previous half cycle is suddenly "dumped" into the branch circuit which it is shunting and which comprises, of course, the second-to-ignite lamp. This spike passes through the filaments of the lamp and will eventually destroy them in a shorter time than warranted by the manufacturer. Thus, the manufacturer has limited the peak current to be passed by requiring that any ballast circuit which is used to operate the lamp must produce a crest factor current wave shape including this spike which does not exceed 1.7. As stated above, the manufacturer also requires that the shunting capacitor have a capacitance of 0.1 microfarad to give the desirable starting and lamp balance characteristics.
There have been instances where high peak currents have occurred in certain ballasts due, it is believed, to the creation of harmonics of the fundamental frequency of the current wave shape resulting from the combination of the shunting capacitor and the secondary winding. The solution to the problem in the past has been to provide additional inductance in the circuit through the use of auxiliary chokes for suppressing the spikes. Such circuits are disclosed, for example, in Hume Pat. No. 3,225,255 and Feinberg et al Pat. No. 3,195,012. Added components increase the cost of the ballast, not only because of the additional copper and steel but also due to the added labor of connecting them into the circuit. Additionally, they increase the size of the ballast.
The invention solves the problems mentioned by providing means for suppressing the current peaks in the second-to-ignite lamp through the incorporation of inductance in the shunting branch which is parallel with the second-to-ignite lamp that requires no substantial changes in the manufacture of the ballast as though it were a conventional ballast. This is done by utilizing a few turns of the secondary winding to serve as an inductance in series with the capacitor shunting the second-to-ignite lamp while these turns are still being used along with the remainder of the secondary winding as the total operating secondary winding for the ballast.